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Unequal Shrinkage.                          69

^ "Warping and Shrinkage of Castings.—The general
effects produced by unequal shrinkage during cooling should
be well understood in designing a casting. These may be pretty
well arrived at by the consideration that, other things being equal,
those places will cool last where the largest amount of metal is
aggregated. Our first rule, therefore, is to endeavour to keep the
casting uniform in thickness. For unequal cooling is sure to
produce internal strains, and that portion cooling jftrst will set, and
be compressed by the contraction of the fart that is still coding*
Besides, if a thin part join a thick part very abruptly, the cooling
may produce such strains as to break the thin piece away
altogether. We ought therefore to make the juncture of unequal
thicknesses as gradual as possible.

Take a Plate, Fig. 78, lying on a surface of sand. The top
part cools first, on account of being open to the air, while the
under surface is still in contact with the hot sand, and the effect
of cooling is to make the plate convex on the upper surface, by
the after contraction of the lower surface.

In a Hollow Cylinder, Fig. 79, the heat cannot pass-
through the core so quickly as it can from the outside, so the
latter cools first, and the cylinder is made barrel-shaped by the
contraction of the interior. We must also note that the outer
layer will be in compression (see Fig. 83), which is a cross section.

A Solid Ball will be found porous on the inside, if broken,
because the shell sets fir^t, and the internal metal, being thus held
fast, is bound to leave vacuities on shrinking.

A Girder of the form sketched in Fig. 80 will curve longitu-
dinally in cooling, for here the most metal is collected in the
larger flange, and the casting is therefore pulled together on that
side, after the top web has cooled.

A Pulley with a thin rim, as in Fig. 81, will cool last at the
centre boss, and so produce a compressive strain in the rirn ; if
therefore a piece were broken out at A it could not be returned.

Shrinkage occurs while the metal is cooling from a red heat
downward, and the moulder can do a great deal to prevent it
occurring unequally by uncovering at the red-hot stage those
portions of the casting which are likely to retain heat longest, and
by keeping others covered, for equal cooling means equal shrinking.